The medieval identity theory

According to identity theory, an affirmative categorical proposition is true only if its subject and predicate terms refer to the same thing or things. On this analysis the proposition ‘Socrates is wise’ for example is true if and only if Socrates, the referent of ‘Socrates’, is one of the wise persons, i.e. the extension of the term ‘wise’. If any of the two terms of an affirmative categorical is ‘empty’, then the proposition in question refers to nothing and therefore is false. According to this theory, as Buridan puts it: “every affirmative proposition whose subject or predicate refers to nothing is false”.